The invention relates to an improved method for injecting treatment chemicals into a flowing stream such as a process stream in a hydrocarbon processing plant. Specifically, the invention comprises the use of continuous and constant injection of a desired treatment chemical into a flowing process stream to insure the optimum effect of the treatment chemical. Conventional, noncontinuous methods of injecting treatment chemicals into process streams reduce the effect of the treatment chemicals due to the intermittent injection and lack of axial dispersion of the treatment chemical in the flowing process streams. This produces a nonuniform concentration of the treatment chemical in the process stream.
The hydrocarbon processing industry, chemical industry, oil production industry, water treatment plants, and other similar industries and plants frequently use relatively small amounts of treatment chemicals to control undesirable occurrences in process streams and other streams in the plants. The undesirable occurrences may take many forms such as corrosion, saltation, fouling, wax formation, scale formation, and polymerization in pipes or equipment. Corrosion, for example, deteriorates the metal in pipes and process equipment and may cause failure of the pipes or equipment. Fouling and wax formation, for example, occur when particular materials are deposited in pipes and equipment due to undesirable chemical processes, and may lead to plugging of the pipes or equipment.
These problems vary in severity from minor annoyances in the operation of a plant to problems that halt operations of an entire plant. For example, a change from a nonacidic crude oil feedstock in an oil refinery to an acidic crude oil feedstock may cause pipes exposed to the acidic component of the crude oil to experience sudden and severe corrosion. The pipes may develop a hole within hours or days, and cause a processing unit or the whole refinery to shut down. Thus, the effective use of the appropriate treatment chemicals to eliminate these problems may be of paramount importance for the operation of a hydrocarbon processing plant or other plant.
Various treatment chemicals are available to remedy each of these problems in any particular application. Many chemical companies manufacture and sell treatment chemicals to alleviate specific problems for particular types of process streams. For example, Nalco Chemical Number 5192 made by Nalco Chemical Company may be used to prevent corrosion in overhead process streams.
Typically, treatment chemicals are injected intermittently into process streams because the pumps used for this purpose provide an intermittent, nonconstant flow of the treatment chemical. Generally, positive displacement pumps are used to inject treatment chemicals into flowing process streams. Several types of positive displacement are available.
Piston pumps are one type of positive displacement pump used for injecting treatment chemicals. For example, a Milton-Roy pump incorporates one or more pistons which are driven by a motor. The pistons pull liquid into a piston chamber when the piston is moving away from top dead center of its stroke, and discharge liquid when the piston is moving towards top dead center of its stroke.
Another type of a positive displacement pump is a diaphragm pump which incorporates a flexible diaphragm as one wall of a fluid chamber. When the diaphragm is flexed away from the chamber liquid is pulled into the chamber by the suction created by the diaphragm. When the diaphragm is flexed into the chamber liquid is discharged from the chamber due to the pressure from the diaphragm. Generally, positive displacement pumps are used to inject treatment chemicals into process streams because of their dependability and their ability to accurately discharge a measured amount of treatment chemical.
Positive displacement pumps share a common flow pattern that derives from their nature and results in a two stage cycle: Stage 1 is a discharge stroke that produces a positive flow of liquid which varies with time and is typically represented by a positive sine wave. Stage 2 is a return stroke during which there is no liquid being discharged. The duration of the discharge stroke and the duration of the return stroke are the same, and the combined duration for both strokes is the cycle time for the pump. The cycle time for positive displacement pumps ranges from about 0.6 to 1 second. Thus, for a positive displacement pump operating at full capacity, liquid is only being discharged during 50 percent of the cycle time at best.
Typically, the amount of liquid discharged by positive displacement pumps may be varied from 10 percent of the pump's discharge capacity to the pump's full discharge capacity. When a positive displacement pump is adjusted to deliver less than its full capacity the pump only discharges liquid during a portion of the pump discharge stroke. And the sine wave representing the flow of liquid from the pump is reduced in amplitude. The result is a decrease in the total amount of liquid discharged by the pump.
The total amount of time that the positive displacement pump does not discharge liquid is the combined amount of time of the return stroke and the amount of time during the discharge stroke when no liquid is being discharged. Consequently, if the pump is operating at less than full capacity, treatment chemicals will be injected into the flowing process line less than 50 percent of the time.
During the cycle time when no treatment chemical is being discharged by a positive displacement pump the liquid of the process stream is continuing to flow past the injection point. This section of liquid is not being treated. With the pump at full capacity the section of liquid with no injected treatment chemical corresponds to the amount of liquid that flows past the injection point during the return stroke. If the pump is operating at less than full capacity, this section of liquid corresponds to the amount of liquid that flows past the injection point during both the return stroke and the portion o the discharge stroke when no liquid is discharged. At a minimum, 50 percent of the liquid in the process stream line will not be injected with treatment chemical. And if the pump is operating at less than full capacity this percentage will be greater than 50 percent.
When treatment chemical is injected intermittently into a process stream the chemical will mix rapidly in a radial direction from the point of injection. Consequently, the concentration of the treatment chemical is relatively uniform across the cross-section of the flowing process stream within a short distance from the point at which the treatment chemical is injected. This is due to the rapid radial mixing that occurs in the turbulent flow regime of most flowing process streams.
Axial mixing, however, does not appear to occur rapidly in a flowing process stream. It is generally a function of the nature of the flowing liquid, the nature of the injected liquid, and the flow regime of the flowing liquid. The nature of the flowing liquid and the treatment chemical are important to the extent that the liquids will tend to mix. For example, if the liquids have some chemical attraction to each other they will tend to mix. In the case of a polar treatment chemical being injected into a flowing polar liquid, the polar affinity between the treatment chemical and the flowing liquid will cause axial dispersion more quickly than would occur for a nonpolar treatment chemical injected into a flowing nonpolar liquid.
The flow regime of a flowing fluid is dependent on the velocity that the fluid is flowing, the geometry of the flow, and the density and viscosity of the flowing fluid at flow conditions. This relationship is calculated as the Reynold's Number of the flowing fluid. The Reynold's Number is a dimensionless quantity that represents the ratio between the inertial forces in a flowing fluid and the viscous forces in a flowing fluid. It is frequently used to correlate various parameters relating to the behavior of flowing fluids.
The Reynold's Number (Re) is calculated by the following mathematical formula: EQU Re=DVp/.mu.
where D is the pipe diameter in feet; V is the liquid velocity through the pipe in feet per second; p is the liquid density in pounds per cubic foot; and .mu. is the liquid viscosity in pounds per foot per second. Robert H. Perry and Cecil H. Chilton, Chemical Engineer's Handbook, McGraw-Hill Book Company, 5th ed., 1973, page 5-4, FIG. 526. For a given flow geometry (e.g. flow in a pipe) empirical data related to the Reynold's number indicates whether the flow regime of a flowing liquid is laminar or turbulent.
Laminar flow occurs at low flow velocities, and is characterized by minimal radial mixing between microscopic elements of the flowing liquid. Further, laminar flow is characterized by different flow velocities for microscopic elements of the flowing liquid depending on the distance between the element of the flowing liquid and the wall of the pipe in which the liquid is flowing. Turbulent flow occurs a high flow velocities, and is characterized by extensive radial mixing and random variations in the flow velocities of microscopic elements of the liquid.
For a liquid flowing in a pipe the flow regime is generally laminar at Reynold's Numbers less than 3000, and turbulent at Reynold's Numbers greater than 3000. Typically, process streams in hydrocarbon processing plants have Reynold's Numbers in excess of 3000 and the liquids are flowing in a turbulent flow regime.
Reported studies have noted the degree to which axial dispersion will occur in flowing liquids in pipes. T. Sherwood, R. Pigford, and C. Wilke, Mass Transfer, McGraw-Hill Publishing Company, 1975, 137-141. These studies generally indicate that axial dispersion of a liquid in another flowing liquid correlates with the Reynolds number of the flowing liquid. Mass Transfer, supra at FIG. 4.17. More particularly the effective axial dispersion coefficient, which is a measure of the tendency for a liquid to axially disperse in another flowing liquid, will increase as the Reynold's Number for the flowing liquid increases.
Overall the concentration profile of a liquid injected into a flowing liquid in a turbulent flow regime will follow a Gaussian curve. Mass transfer, supra at 138 and FIG. 4.16. Very little dispersion will occur at a point near the point of injection, and dispersion will gradually increase as the liquid flows farther from the point of injection.
For example, for two batches of oil flowing through a 12-inch pipeline at a velocity of 4 feet per second, the second batch of oil will only disperse 750 feet into the first batch of oil after traveling 24 miles through the pipeline. Mass Transfer, supra at p. 140-41.
Referring to EXAMPLE 1 a test flow loop was constructed to study axial dispersion in a liquid flowing through a tube. Using a diaphragm pump, which provided an intermittent injection of red dye, it was observed that minimal dispersion of the red dye occurred 50 feet from the point of injection of the red dye into a flowing water stream. Further, large sections of the flowing water stream had no observable concentration of the red dye at all.
If this effect is scaled up to the size of typical plant streams it is evident that significant portions of a process stream will not contain any concentration of a treatment chemical. For example, consider an overhead line in a crude oil processing unit with a 10 inch diameter which carries a flowing liquid with a velocity of 100 feet per second. A positive displacement pump is used to inject a treatment chemical such as a corrosion inhibitor. The positive displacement pump is operated at 25 or 30 percent of its capacity because these pumps are typically sized to provide maximum extra capacity.
If the pump operates at 1 cycle per second and is adjusted to deliver 25% of its capacity the treatment chemical will only be injected for 1/8 of a second. The time period of no injection will be 7/8 of a second. During the injection period of 1/8 of a second the flowing process stream will move 12.5 feet, and a section 12.5 feet long will contain the treatment chemical. During the period of no injection the flowing process stream will move 87.5 feet and a section 87.5 feet long will contain no treatment chemical. Five seconds later the flowing process stream will have traveled 500 feet. At which time, based on the flow loop test, the treated section will have slightly expanded from 12.5 feet and the untreated section will have slightly decreased from 87.5 feet.
The combined effect of intermittent injection of a treatment chemical into a flowing process stream and the lack of axial dispersion of the treatment chemical in the process stream is that significant portions of the flowing process stream will have no concentration of the treatment chemical. This problem increases as the velocity of the flowing process stream increases relative to the cycle time of the pump used to inject the treatment chemical because the amount of nontreated process stream correspondingly increases. Thus, the effectiveness of the treatment chemical is reduced. In fact, the treatment chemical may not provide any benefit at all in these situations. Consequently, there is a need for a method that provides a continuous and constant injection of a treatment chemical into a flowing process stream.